Support validation of nested collections in Microsoft.Extensions.Validation

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Today, when we encounter a collection, we simply unwrap it and try to get validatable type info for its direct elements. We don't support nested collections like `Model[][]` or `List`, etc.

### Describe the solution you'd like

Support nested collections.

### Additional context

_No response_

No files or tests are named. Start by tracing how Microsoft.Extensions.Validation unwraps collections and obtains validatable type information; done means nested cases such as Model[][] and List are validated correctly, with focused tests covering both examples.

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